[ipfs/go-bitswap] The splitter should scale with the wantlist size.

The request splitter currently splits requests into `split` pieces. However, we _really_ want to send each wantlist entry to `N` peers. These two are equivalent when the wantlist size stays the same but the wantlist size doesn't.

If we go from a wantlist size of 4 to 32, we'll still have a split of 4 and get 24 duplicate blocks.